Having a high calcium level, indicated by a blood test, is normal for reproductively active (laying) ducks. It doesn't necessarily indicate a problem. I get preventative blood work done on my ducks regularly and my very experienced avian vet always says something like, "their calcium levels are above normal, but that's to be expected since they are both laying."
I'm sure at some point it would be too high - and I'm not sure what that would be since I pay a vet to interpret the lab work for me.
Also, when my ducks were younger they would give an egg every day all spring, summer, and fall. So... Maybe your vet is thinking more about wild mallard egg laying patterns?
Regarding the dark room, I have done that with one of my ducks, under the guidance of my vet, and it worked. My duck was very sick and it provided immediate relief. We basically turned their barn room into a blackout room. We sealed every crack. We gave all the ducks a 10 hour daylight schedule year around by putting a timer on a light inside the barn, and putting them to bed on time every night. They also can't have ANY noise that keeps them up. My vet told me that sometimes it works and sometimes it doesn't. After a season we stopped, and my duck's symptoms returned, so we had to do this for the remainder of her life.
We also removed all eggs, real or dummy, and even egg shaped/sized rocks. We destroyed nests and dark cubbies. All these things encourage egg production.
We moved all the duck items around the aviary and barn regularly. This makes them feel like it is not a secure place to raise babies. So, put the food bowls/water buckets/bathing pool/storage containers in different spots as often as you can.
You might want to get the ok from a professional on this one. I never would have done this, except my vet told me to and my duck was going to die if she didn't get better. We fed her only scratch, as much as she could eat, but just 3 times a day and then picked up the food. She was allowed to forage. I gave her only one mealworm a day. The theory is that if food isn't plentiful it isn't a good time to have babies. She had frequent vet checks and blood work - vitamin injections at the vet and oyster shells free choice at home.
Sorry this is so long.... But this is what worked for me and it is super detailed. I had another duck who had the hormonal implant and injection and that works too.